1. Field of the Invention
The present invention relates to a diffractive optical element having such a grating structure that a light flux of a useful wavelength region concentrates at a specific order (design order), and to an optical system having the diffractive optical element.
2. Description of Related Art
While it has been practiced to reduce chromatic aberration of an optical system by combining different lens materials, another method for reducing chromatic aberration by arranging, on a lens surface or within an optical system, a diffractive optical element (or a diffracting grating) having a diffracting function was disclosed in optical literature, such as xe2x80x9cInternational Lens Design Conference (1990)xe2x80x9d, SPIE Vol. 1354, etc., and the publications of Japanese Laid-Open Patent Applications No. HEI 4-213421 and No. HEI 6-324262 and U.S. Pat. No. 5,044,706. This method is based on the physical phenomenon that the direction in which chromatic aberration takes place for rays of light of a certain wavelength region on a refractive surface becomes inverse to that on a diffractive surface.
Comparing a refractive surface (lens surface) with a diffractive surface with respect to their effect on rays of incident light, one ray of light remains one after refraction on the refractive surface, whereas one ray of light is split into a plurality of rays of different orders when it is diffracted by the diffractive surface. Therefore, in using a diffractive optical element, the structure of the grating of the diffractive optical element is decided in such a way as to cause a light flux of a useful wavelength region to concentrate at a specific diffraction order (hereinafter referred to as the design order). With the light flux concentrating at a specific diffraction order, such as a + first order or a xe2x88x92 first order, rays of diffraction light of orders other than the specific diffraction order have a low degree of intensity. When the intensity of the rays becomes zero, the diffraction light ceases to exist.
In enhancing the efficiency of diffraction for a diffraction light of an m-th order, a phase difference giving structure is arranged to give a phase difference of 2 xcfx80m to rays of each optical path in the diffracting direction. The rays of light are then caused by this arrangement to interfere with each other and are thus intensified.
FIG. 12 shows a structural arrangement of a transmission type diffractive optical element 1. In the diffractive optical element 1, the grating thickness of a diffraction grating 3 is assumed to be d and the refractive index of the material of the diffraction grating 3 is assumed to be n. In order to give the phase difference of 2 xcfx80m to light of a wavelength xcex of an m-th order of diffraction, the structural arrangement is required to satisfy the following condition:
2 xcfx80m=2 xcfx80d(nxe2x88x921)/xcexxe2x80x83xe2x80x83(1). 
In a case where the condition of the formula (1) above is established at each pitch of the grating, the diffraction efficiency becomes higher.
The actual structure of the diffractive optical element which is necessary for attaining this diffracting function is called a kinoform. There are various known modes of arranging the kinoform structure. In one known mode, spans for which the phase difference of 2 xcfx80m is given are arranged to continue one after another. In another known mode, a continuous phase difference distribution of kinoform is approximated stepwise in a binary shape. In a further known mode, a minute periodic structure of kinoform is approximated in a triangular wave shape. Each of these structures is arranged either on the surface of a flat plate or on a convex or concave lens surface within an optical system. Further, the diffractive optical element of this type is manufactured, for example, with a mold prepared by a semiconductor manufacturing process such as lithography or by machining or with a replica formed on the basis of such a mold.
The diffractive optical element is capable of greatly reducing chromatic aberration taking place on a refractive surface due to dispersion by a glass material. The diffractive optical element can be arranged to have a great aberration reducing effect, like an aspheric lens, by varying the period of its periodic structure.
In the case of the prior example mentioned above, various aberrations, particularly a chromatic aberration, are lessened by the effect of diffraction. The effect attained by including the diffractive optical element in an optical system can be confirmed, for example, on a drawing showing aberrations or the like. However, if the diffraction efficiency is not high for the diffraction light of the design order, no light might be existing there in actuality. It is, therefore, necessary to have a sufficiently high diffraction efficiency for diffraction light of design order. Further, in a case where there is any light having diffraction orders other than the design order, that light is imaged at a different part from where the light of the design order is imaged. Such a light thus becomes flare light to lower the contrast of an image. Therefore, it is important, for any optical system that is designed to utilize a diffraction effect, to sufficiently consider a spectral distribution of diffraction efficiency and the behavior of light of diffraction orders than the design order.
FIG. 13 shows the spectral characteristic of diffraction efficiency obtained for a specific diffraction order with the diffractive optical element shown in FIG. 12 formed on a certain surface within an optical system. In FIG. 13, the abscissa axis indicates wavelength and the ordinate axis indicates the diffraction efficiency. The diffractive optical element is designed to have the diffraction efficiency become highest at the first order of diffraction (shown in a full line curve in FIG. 13). In other words, the design order is the first order. FIG. 13 further shows the diffraction efficiency for diffraction orders near the design order, i.e., the zero order and the second order ((1xc2x11)-th order). As shown in FIG. 13, at the design order, the diffraction efficiency becomes highest at a certain wavelength (hereinafter referred to as a xe2x80x9cdesign wavelengthxe2x80x9d) and gradually decreases at other wavelengths. The reason for this is as follows. The grating thickness required for making the phase difference 2xcfx80 is as expressed by the formula (1). In a case where the grating thickness is set to satisfy the condition of this formula for the design wavelength, this condition becomes somewhat unsatisfied for other wavelengths, thereby causing a drop in diffraction efficiency.
The drop portion of the diffraction efficiency at the design order becomes diffraction light of other orders and comes to appear as flare light. In a case where the diffractive optical element is provided with a plurality of diffraction gratings, the drop in diffraction efficiency at wavelengths other than the design wavelength eventually causes a decrease in transmission factor.
In view of the above problem, the inventor of the present invention has developed a diffractive optical element, as disclosed in Japanese Patent Application No. HEI 8-307154, which has a grating structure as shown in FIG. 18. In the grating structure shown in FIG. 18, a plurality of diffraction gratings including a first diffraction grating 3a and a second diffraction grating 3b, which are made of at least two kinds of materials which differ in dispersion, overlap each other. With the diffractive optical element arranged in this manner, its diffraction efficiency remains high at the design order over the whole region of useful wavelengths, as shown in FIG. 19.
Another diffractive optical element formed by overlaying on each other diffraction gratings of materials of two different kinds was disclosed in U.S. Pat. No. 5,017,000, etc. This optical element, however, relates to a multiple focus lens and nothing has been disclosed with respect to how to enhance its diffraction efficiency.
Further, the publications of Japanese Laid-Open Patent Applications No. HEI 9-127321 and No. HEI 9-127322 have disclosed diffractive optical elements arranged to prevent color fluctuations and generation of flare light due to light of unnecessary diffraction orders by lowering the wavelength dependency of the diffraction efficiency. More specifically, the diffractive optical element is formed by laminating a plurality of different optical materials (two or three optical materials) with one or two relief patterns formed at the boundary face between the different optical materials.
In the diffractive optical element disclosed in the above Japanese Laid-Open Patent Application No. HEI 9-127321 or No. HEI 9-127322, there are two wavelengths at which the phase amplitude becomes xe2x80x9c1xe2x80x9d, as shown in FIG. 15. The diffractive optical element is thus arranged, on the basis of these wavelengths, to have two optimized wavelengths (design wavelengths) where the maximum diffraction efficiency can be obtained. FIG. 16 shows the diffraction efficiency obtained at the design order, and FIG. 17 shows the diffraction efficiency obtained at diffraction orders in the neighborhood of the design order. Since there are two design wavelengths, the diffraction efficiency trends downward at either of two ends of the useful wavelength region of 400 nm to 700 nm. In the case of FIG. 16, the diffraction efficiency drops to a level of 94% or thereabout on the side of longer wavelengths. Then, in inverse proportion to the diffraction efficiency shown in FIG. 16, the diffraction efficiency obtained at diffraction orders in the neighborhood of the design order increases up to 2% or thereabout on the side of longer wavelengths, as shown in FIG. 17.
Therefore, the use of the diffractive optical element, under special service conditions, for the useful wavelength region of 400 nm to 700 nm has not been always satisfactory for reducing the amount of generated flare light due to light of unnecessary orders. The diffractive optical element thus has been desired to have a higher diffraction efficiency over the useful wavelength region. The above-stated special service conditions include, for example, a case where the diffractive optical element having the above-stated diffraction efficiency is applied to a photo-taking lens of a camera or the like. In the case of the camera, a film is used on an evaluation plane and there are various photo-taking conditions (object and exposure conditions) occurring. Among such various conditions, in the event of, for example, a light source of a high degree of luminance existing at a part of the object, the high luminance light source causes an exposure of the film to occur that is saturated more than an exposure appropriate to the film while an apposite exposure is adjusted to other parts of the object in taking a shot. In that event, since the exposure for the light source is several times more than the apposite exposure, even a slight amount of diffraction light of orders near the design order might be multiplied several times. Then, the slight amount of diffraction light tends to result in flare light around the light source, like a halo.
It is an object of the invention to provide a diffractive optical element, or an optical system having the diffractive optical element, which has a high diffraction efficiency over the whole useful wavelength region with the amount of unnecessary diffraction light made small.
To attain the above object, in accordance with an aspect of the invention, there is provided a diffractive optical element, which is formed by laminating at least three layers of diffraction gratings made of at least three kinds of materials which differ in dispersion, wherein at least three design wavelengths are set.
The diffractive optical element according to the invention has the following features.
In the diffractive optical element according to the invention, the above-mentioned at least three layers of diffraction gratings are laminated on a base plate, and, letting the diffraction gratings be assumed to be a first diffraction grating, a second diffraction grating and an L-th diffraction grating, as counted from the base plate, a grating thickness of the first diffraction grating be denoted by d1, a grating thickness of the second diffraction grating be denoted by d2, a grating thickness of the L-th diffraction grating be denoted by dL, a refractive index of the first diffraction grating at a wavelength xcex be denoted by n1(xcex), a refractive index of the second diffraction grating at the wavelength xcex be denoted by n2(xcex), a refractive index of the L-th diffraction grating at the wavelength xcex be denoted by nL(xcex), an arbitrary wavelength within a useful wavelength region be denoted by xcex0, and a design order of diffraction be denoted by m, it is preferable to satisfy the following condition:
0.9217xe2x89xa6{(n1(xcex0)xe2x88x921)d1xc2x1(n2(xcex0)xe2x88x921)d2xc2x1 . . . xc2x1(nL(xcex0)xe2x88x921)dL}/mxcex0xe2x89xa61.0783. 
Further, in the diffractive optical element according to the invention, the above-mentioned at least three layers of diffraction gratings are laminated on a base plate, and, letting the diffraction gratings be assumed to be a first diffraction grating, a second diffraction grating and an L-th diffraction grating, as counted from the base plate, a grating thickness of the first diffraction grating be denoted by d1, a grating thickness of the second diffraction grating be denoted by d2, a grating thickness of the L-th diffraction grating be denoted by dL, a refractive index of the first diffraction grating at a wavelength xcex be denoted by n1(xcex), a refractive index of the second diffraction grating at the wavelength xcex be denoted by n2(xcex), a refractive index of the L-th diffraction grating at the wavelength xcex be denoted by nL(xcex), an arbitrary wavelength within a useful wavelength region be denoted by xcex0, and a design order of diffraction be denoted by m, it is preferable to satisfy the following condition:
0.8755xe2x89xa6{(n1(xcex0)xe2x88x921)d1xc2x1(n2(xcex0)xe2x88x921)d2xc2x1 . . . xc2x1(nL(xcex0)xe2x88x921)dL}/mxcex0xe2x89xa61.1245. 
In a mode of the diffractive optical element according to the invention, the design wavelengths include at least two design wavelengths which are shorter than a center wavelength of a useful wavelength region.
In another mode of the diffractive optical element according to the invention, the useful wavelength region is a visible spectrum.
In a further mode of the diffractive optical element according to the invention, the diffraction gratings made of at least three kinds of materials which differ in dispersion include at least one kind of grating structure whose grating thickness within one period monotonously increases between materials which differ in dispersion and at least one kind of grating structure whose grating thickness within one period monotonously decreases between materials which differ in dispersion.
In a still further mode of the diffractive optical element according to the invention, the first diffraction grating and the base plate are made of the same material.
Further, there is provided an optical system as a part of which one of the above diffractive optical elements according to the invention is used. The optical system includes, in particular, an image forming optical system and an observation optical system.